The present invention relates generally to the cooling and sealing air system in turbomachinery. In particular, the invention relates to a mixer used in the cooling and sealing air system in turbomachinery.
The cooling and sealing air system in a turbomachine (e.g., a turbine) utilizes air from the compressor (e.g., an axial flow compressor) to: (1) cool internal parts of the gas turbine, (2) cool the turbine outer shell and exhaust frame, (3) seal the turbine bearings, (4) prevent compressor pulsation, and (5) provide an air supply for air operated valves.
The following example describes some of the functions of the cooling and sealing air system in an exemplary gas turbine, and is not intended to limit the scope of the present invention in any way. During the startup sequence of a gas turbine, air is extracted from the 9th and 13th stages of the axial flow compressor and is vented to atmosphere. These 9th and 13th stage extractions, together with the inlet guide vanes in the closed position, limits airflow through the compressor and prevents compressor pulsation.
During normal operation of the gas turbine, air from the 9th and 13th stages is used for cooling and sealing 2nd and 3rd stages of the gas turbine. This is achieved by means of external piping and flow controlling orifices sized to meet the worst case operating requirements of the gas turbine, typically the cold and hot day operating conditions. The pressurized air entering the 2nd and 3rd stage casing manifolds purges the hot turbine air from the 2nd and 3rd stage wheel space cavities.
While the turbine is being unloaded or shut down, the cooling and sealing air system continues to cool and seal the turbine wheel space cavities and the interior turbine components. Air from the 9th and 13th stages of the compressor is again vented through the exhaust plenum. This prevents compressor pulsation during the turbine deceleration period.
The diverted air from the axial flow compressor can consume a large proportion of the total air flow through the compressor, for example, as much as 20%. The management and control of these parasitic flows can dramatically increase the performance of the turbine. The extraction ports often provide cooling air flow at too high a pressure and/or temperature and typically the flow is throttled, resulting in a net loss of energy. By employing an ejector, the low pressure/temperature flow (e.g., from the 9th compressor stage) may be mixed with the high pressure/temperature flow (e.g., from the 13th compressor stage) to provide a flow at an intermediate pressure and temperature substantially matching the pressure and temperature required to cool a turbine component, while simultaneously making use of low pressure and temperature air which otherwise might be dissipated as wasted energy.
An ejector in a cooling and sealing air system of a gas turbine helps in reducing the usage of expensive (i.e., in terms of work expended) high-pressure air by replacing it with relatively inexpensive low-pressure air. The ratio of the mass flow of low-pressure air pumped by the ejector by expending a pound of high-pressure air is called the entrainment ratio. For maximum benefit of the ejector system, a high entrainment ratio is expected over all operating conditions of the gas turbine. Depending on the cooling flow requirement of the turbine, bypass flow is needed at several operating conditions. A high entrainment ratio improves overall gas turbine performance (both efficiency and output).
Ejectors, however, have no moving parts and are designed for operation at specific design points based on ISO day conditions. ISO standard day conditions are 59° F., 14.7 psia, 60% relative humidity or 15° C. (288° K.), 101.3 kilopascals, 60% relative humidity. For turbine applications, the turbine inlet conditions to the ejector are a function of ambient day conditions in which the turbomachinery operates. The ambient day variations seen by the gas turbine can vary, for example, from −20° F. to +120° F., which results in about a 50% temperature and about 50% pressure variation on the inlet/exit conditions to the ejector. This variation has a strong effect on the operational characteristics of the ejector to the extent that, at many ambient day conditions, the ejector will not provide adequate cooling and/or purge flow. That is, the ejector behaves differently on different days and at different times during each day, and on certain days, the ejector will provide insufficient benefit.